Implantable cardioverter-defibrillators (ICDs) are used to provide various types of therapy to a cardiac patient, including, for example cardioversion and/or defibrillation. These devices consist of a hermetic housing implanted into a patient and connected to at least one defibrillation electrode and with at least one other electrode e.g., a patch-type electrode, a housing- or can-based electrode, a surface-type electrode, and a stent-based electrode) thereby defining a therapy vector between various pairs of said electrodes. The housing of the ICD contains electronic circuitry for monitoring the condition of the patient's heart, usually through sensing electrodes, and also contains the battery, high voltage circuitry and control circuitry to generate, control and deliver the defibrillation shocks. Typically, one or more specialized defibrillation-type or other transvenous leads are connected to circuitry within the ICD and extend from the housing to one or more defibrillator electrodes proximate the heart. The housing of the ICD may include one or more defibrillation electrodes configured on the exterior of the housing. One example of an ICD is disclosed in U.S. Pat. No. 5,405,363 to Kroll et al., the disclosure of which is hereby incorporated by reference.
Dislodgement of a right ventricular (RV) defibrillation lead to the right atrium (dislodgement to the atrium) is a rare complication associated with ICD therapy. However, such dislodgement deserves attention out of proportion to its low incidence because it may cause fatal proarrhythmia.
Under certain circumstances, dislodgement of an ICD lead into the atrium may cause fatal proarrhythmia in a patient having a normal physiological sinus rhythm. For example, a patient having an ICD may exercise and increase the heart rate to 120 beats-per-minute (bpm) corresponding to an R-R interval of 500 ms and a P-R interval of 200 ms. Both the P-waves and the R-waves are sensed with alternating P-R and R-P intervals of 200 ms (equivalent to 300 bpm) and 300 ms (equivalent to 200 bpm), resulting in detection of ventricular fibrillation (VF) by the implanted cardioverter-defibrillator.
In response to erroneously detecting VF, a shock synchronized to the atrial electrogram (EGM) will likely be delivered by the ICD during the ventricular vulnerable period, 300 ms after the preceding oversensed P-wave. The vector of this shock is from the coil on the right ventricular (RV) lead to the housing or can of the ICD. But because the RV lead has dislodged, the coil of the RV lead is now likely positioned within the right atrium. This vector is sufficient for cardioversion of atrial fibrillation, but not for ventricular defibrillation. Thus the shock will likely be below both the ventricular upper limit of vulnerability, and the defibrillation threshold for this shock vector. Because of this, the shock has a high likelihood of actually inducing VF that the ICD cannot defibrillate.
In another circumstance, dislodgement of an ICD lead into the atrium may cause fatal proarrhythmia in a patient experiencing atrial fibrillation (AF). For example, the high rate of the AF is falsely classified by the implanted cardioverter-defibrillator as VF. In response, a shock synchronized to the atrial electrogram will likely be delivered by the ICD during the ventricular vulnerable period. The vector of this shock is from the coil on the RV lead to the housing or can of the ICD. But because the RV lead has dislodged, the coil of the RV lead is now likely positioned within the right atrium. Because the shock vector is inefficient (right atrium to ICD housing or “can”), the shock's strength will likely be below both the ventricular upper limit of vulnerability and the ventricular defibrillation threshold. Thus the shock has a high likelihood of inducing VF that the ICD cannot defibrillate.
If the inappropriate shock from the dislodged lead defibrillates (cardioverts) the atrium and the ICD then senses only the atrial signals of normal rhythm from its sensing channel, it will classify the shock as successful and, despite ongoing VF, not deliver another shock.
Further, lead dislodgement to the atrium presents a significant risk even if no inappropriate shock is delivered because the ICD is unlikely to defibrillate spontaneous VF with this shock vector and diagnosis of lead dislodgement may be delayed because no present ICD features focus on rapid diagnosis.
Presently, no ICD has implemented any proposed method or algorithm to detect or mitigate lead dislodgement to the atrium. The only proposed solution to the problem of lead dislodgement to the atrium is insufficient. US Published Application No. 2014/0018873 to Gunderson (“the '873 Publication”), teaches an algorithm that withholds therapy in sinus rhythm based on an anticipated pattern of electrical signals on the ventricular near-field (NF) EGM. In general, the term “near-field” refers to an EGM recorded by two or more electrodes, all of which are located in proximity to the source signal for the EGM. As used in ICDs, the NF-EGM ventricular is recorded from two closely-spaced electrodes near the tip of the lead, at least one of which is a small sensing electrode at the tip of the lead. Because these electrodes are closely spaced, their electrical “field of view” is short-range and dominated by the electrical signals originating in myocardium adjacent to the lead tip. The NF-EGM is thus ideal for sensing local myocardial electrical activity, and all ICDs monitor the NF-EGM continuously for the purpose of sensing the cardiac rhythm.
In contrast, a FF-EGM is an EGM recorded by one or more electrodes located at a distance from the source of the EGM. A ventricular FF-EGM records ventricular activation using at least one electrode that is not in a ventricle. As used in ICDs, the ventricular FF-EGM usually refers to an EGM recorded between two or more large, widely-spaced electrodes, used to deliver defibrillation shocks, at least two of which have opposite polarity during the shock and are thus separated in space by a distance of 10 cm or more. The FF-EGM records a more global signal than the NF-EGM, which—as noted previously—records a local signal. The electrodes are commonly used in the art to record the FF-EGM include the right-ventricular defibrillation coil, the housing of the ICD, and (in dual-coil defibrillation leads), the proximal defibrillation coil. Thus, the most commonly-recorded FF-EGM is the “shock” EGM recorded between two or among three widely-spaced, large shock electrodes. However, the EGM recorded between the small electrode at the tip of the defibrillation lead and the housing of the ICD also comprises a FF-EGM, although it is rarely used by ICDs. Under routine operation, the FF-EGM is not monitored continuously or even intermittently during normal rhythm in any ICD. The primary use of the far-field electrical pathway is to deliver high-voltage shocks, not monitor electrical signals. In some ICDs, the FF-EGMs recorded from this electrical pathway is analyzed to perform a secondary function that is activated only after analysis of the sensed NF-EGM indicates that VT or VF is present. This secondary function is to confirm the presence of VT or VF as indicated by the NF-EGM primary sensing channel.
The '873 Publication teaches detection of lead dislodgement to the atrium by the recording of short-long-short-long (S-L-S-L) sequences of intervals between NF-EGM signals. The “short” interval corresponds to the P-R interval; the “long” corresponds to the R-P interval. Additionally, the algorithm requires that each signal have a relatively low amplitude (e.g., 0.5-2.5 mV) and that a zero crossing occurs in the short interval to exclude R-wave double-counting. This algorithm alerts when two such sequences occur. The sensitivity of this pattern for lead dislodgement to the atrium is unknown. Furthermore, there is currently no available evidence that the algorithms described in the '873 Publication perform sufficiently well to be implemented in an implanted ICD.
However, the algorithm described in the '873 Publication would not apply under a number of lead dislodgement to the atrium conditions that do not result in S-L-S-L sequences on the NF-EGM. (1) One example occurs when the atrial rhythm is AF so there are multiple atrial EGMs for each ventricular EGM. Other examples relate to the limited “field of view” NF-EGM. Because this field of view is restricted to local myocardial electrical signals, it does not reliably record signals from two cardiac chambers (atrium and ventricle) simultaneously during the unpredictable conditions of lead dislodgement to the atrium. However, the S-L-S-L sequences on the NF-EGM required by the '873 Publication depend on recording signals from the atrium and ventricle simultaneously. For instance, (2) the lead dislodges but the lead tip remains in the ventricular and does not reach the tricuspid valve so the NF-EGM records a ventricular signal but no atrial signal. (3) The ICD lead dislodges fully into the atrium with the lead tip adjacent to atrial myocardium so that the NF-EGM records an atrial signal but no ventricular signal. (4) The lead is ejected forcefully by ventricular contraction so that its tip moves rapidly from the ventricle into the atrium far from the atrioventricular junction; thus atrial and ventricular EGMs are never both sensed on consecutive cardiac cycles on the so that two S-L-S-L sequences are not recorded; alternatively, a lead dislodging from atrium to ventricle commonly stimulates premature beats, which alters the timing of atrial and ventricular signals so that no S-L-S-L sequence is recorded as the lead traverses the tricuspid valve. (5) As another example, non-capturing ventricular pacing pulses (which are common in lead dislodgement to the atrium) interrupt the S-L-S-L sequence because they are counted as sensed ventricular events. Published examples exist of conditions (1)-(3) during spontaneous lead dislodgements that resulted in unnecessary shocks, and conditions (4)-(5) have been reproduced experimentally. Thus the method of the '873 Publication is insufficient to identify lead dislodgment to the atrium reliably.
Prior art also includes a proposed solution to the reverse problem of atrial lead dislodgement in pacemakers and ICDs that have an atrial lead. U.S. Pat. No. 5,713,932 to Gillberg et al. discloses a method of diagnosing atrial lead dislodgement to the ventricle limited to patients who have intact atrioventricular conduction. Atrial lead dislodgement to the ventricle is diagnosed if the atrial lead is paced and the interval from the atrial pacing pulse to the ventricular NF-EGM is less than the expected delay from atrioventricular conduction.
In view of the above limitations of current approaches, a need remains for an improved method of detecting lead dislodgement.